1. Technical Field of the Invention
The present invention relates generally to a fuel distillation property determining apparatus designed to determine the distillation property of fuel used in diesel engines and a method of determining such a fuel distillation property.
2. Background Art
Modern fuel injection systems for automotive diesel engines are designed to perform a multi-injection control mode in which a sub-injection(s) of fuel is executed preceding or following a main injection for reducing combustion noise and NOx emissions.
It is essential for ensuring beneficial effects of the multi-injection control mode to measure the distillation property (especially, the cetane value also called cetane number) of used fuel such as light oil to correct the quantity of fuel to be injected into the engine or the injection timing thereof.
Japanese Patent First Publication No. 2005-23856 teaches a fuel distillation property measuring method of sampling the speed of the diesel engine for a given number of cycles to determine a difference between a maximum and a minimum speed (which will also be referred to as an engine speed variation below) and determining the distillation property of the fuel based on the engine speed variation. Specifically, when the engine speed variation is greater than a given reference value, the fuel is determined to be low volatile. Alternatively, when the engine speed variation is lower than the given reference value, the fuel is determined to be high volatile.
Japanese Patent First Publication No. 2006-226188 teaches use of an in-cylinder pressure sensor to measure the pressure in a cylinder of the diesel engine to calculate the time when the fuel has been ignited and the amount of burned fuel (i.e., the quantity of heat that is a parameter representing engine output torque) in the sub-injection mode for determining the cetane value of the fuel based thereon.
Usually, the diesel engines are subjected to a great change in speed thereof depending upon the temperature of ambient air or the property of air sucked into the cylinder of the engine even when a constant quantity of fuel is sprayed cyclically into the engine without changing the distillation property of the fuel.
The method, as taught in the former publication, therefore, encounters the drawback in that it is difficult to determine whether the change in speed of the diesel engine is caused by a change in distillation property of the fuel or another factor accurately.
The in-cylinder pressure sensor is usually used for measuring the pressure in the cylinder of the engine in the main injection mode and thus unsuitable for measuring a change in pressure in the cylinder when a small quantity of fuel (approximately 1/30 of that in the main injection mode) is sprayed, for example, in the sub-injection mode to determine whether the pressure change has resulted from the burning of the fuel or some electrical noise added to the output of the in-cylinder pressure sensor. It may, therefore, be impossible to the ignition timing or the amount of burned fuel in the sub-injection mode. The system, as taught in the latter publication, has also the problem that it is difficult to determine the fuel distillation property accurately.